The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

This provision was intended so that if the Soviet Union launched an attack against the European allies of the United States, it would be treated as if it was an attack on the United States itself. However the feared Soviet invasion of Europe never came. Instead, the provision was used for the first time in the treaty's history on September 12, 2001 in response to the September 11, 2001 Terrorist Attack.

The Soviet Union and its satellite states formed the Warsaw Pact in the 1950s in order to counterbalance NATO. Both organisations were opposing sides in the cold war.
Before 1967, the NATO's headquarters was in Paris (France).
Since October 16, 1967 the headquarter has been in Brussels (Belgium).